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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

GLACIERS OF PASSAGE CANAL AND BLACKSTONE BAY        367
of several feet. In the northeast corner there is a circular opening that shows a clear, blue ice. At the bottom of this we discovered two caves running for an unknown distance into the ice. In front of this opening snow has drifted to a depth of SO to 50 feet."
The preservation of such a detached ice mass for a period of over twelve years, as a result of the protection afforded by a thin cover of ablation moraine, is a matter of considerable interest.
A Cascading Glacier. The ice tongue east of Portage Glacier is a cascading glacier, heading in snowfields from which also the southern tributaries of Portage Glacier descend northward, and the CarroD, Concordia, and Milton Glaciers of Blackstone Bay, eastward. It descends part way down the fiord wall and terminates several hundred feet above sea level three fourths of a mile from the shores of Passage Canal. Men-denhall described it as a glacier which "spills over the mountain rim 2000 feet above tide in an ice cataract," and in 1910 it was much the same. A number of small streams with waterfalls drain this cascading glacier and are still building extensive outwash gravel deposits, which end in a projecting delta on the south side of the fiord, near its head. Around the borders of tTn'a cascading glacier, which is clean and severely crevassed, is a moderate-sized barren zone, showing that the glacier has recently been more extensive than at the present time. There are several small ice masses ending high on the mountain slope east of this cascading glacier.
Glacial Erosion in Passage Canal—Fiord Walls. That Passage Canal has been intensely glaciated is proved by the pronounced glacial smoothing and striation. The northern wall, near the head, has broad, eastward-descending rock benches (PI. CXLIU), 2000 feet or more above sea level, showing a minimum height to which it has been glaciated, and proving that the ice moved eastward in Passage Canal. The northern side of the fiord, from Portage Glacier to Blackstone Bay, is much steeper than the southern, which has projecting, untruncated spurs and some islands, suggesting that the main ice stream moved with more power on the northern side. There is no essential difference in the degree of oversteepening of the northern fiord wall to the east and west of Seth Glacier cove, although the walls in the portion to the east are granite, and in the portion to the west graywacke and slate.
Cirques and Hanging Vatteys. The cirque east of Seth Glacier is one of several such forms in Passage Canal, excavated by glacial erosion. The cascading glacier east of Portage Glacier, and several others near it, are in hanging valleys. The valley of Billings Glacier is apparently cut down to sea level but may have a discordant relationship to the fiord bottom. Seth Glacier is in a hanging valley, in the lip of which the stream has cut a deep rock-walled gorge.
Through Valley. The most conspicuous work of glacial erosion above sea level is in the through valley occupied by Portage Glacier (PL CXLIV). This has been cut well down toward sea level, forming the only low pass across the Kenai Mountains, whose other passes are 8000 feet or more high, and whose snowfields and peaks rise to a general elevation of five or six thousand feet.
Submarine Topography of the Fiord. The fiord is 676 feet deep near its head at a point within half a mile of the outwash gravel plain of Portage Glacier (Fig. 64), and the bottom slopes at an average rate of 60 feet to the mile. It is 1176 feet deep near the entrance of Blackstone Bay, ten miles to the east. Eight miles farther east, outside the entrance of Port Wells (Fig. 49), the depth is 1428 feet, the bottom slope having